1. Field of the Invention
This invention relates to a threaded joint for oil-well pipe, especially of the type to which grease according to the API (American Petroleum Institute) Standard is applied, that assures secure sealing under low stresses irrespective of the amount and application method of grease.
2. Description of the Prior Art
Generally, thread compound grease according to the API Standard is applied to the threads of joints meshed together for the purpose of lubrication and for assuring the desired liquid tightness. This practice is not limited to the threaded joints for oil-well piping according to the API Standard but is widely adopted with other special joints having metal-to-metal contact seals.
An example of a threaded joint shown in FIG. 1 consists of a box 1 and a pin 3 and has a metal-to-metal contact seal 5. As the metal-to-metal contact seal 5 provides the desired sealing, threaded portions 7 are not required to perform a sealing function. When used in a sulphide-polluted corroding environment, this type of threaded joints might undergo corrosion cracking under the influence of circumferential stress. To prevent such cracking, it has been long sought to reduce the amount of stress to which the box 1 is subjected. With conventional joints for oil-well pipes having the metal-to-metal contact seal 5, however, thread compound grease applied for the prevention of joint seizure fills the voids in the threaded portion 7 when the box and pin are meshed together. Experimentally, the grease was found to build up as high a pressure as 500 kg/cm.sup.2, which remained substantially unchanged with time.
FIG. 2 graphically shows a change in grease pressure while the box and pin of a conventional threaded joint (with a pipe having an outside diameter of 178 mm and a void between the threads having a cross-sectional area of 0.10 mm.sup.2) are being meshed together at (a) and a change in grease pressure after they have been meshed together at (b). The graphs evidence the grease pressure changes just as described above. In the graphs, solid lines show the grease pressures at a point closer to the metal-to-metal contact seal and dashed lines show those at a point distant from the metal-to-metal contact seal. In actual oil wells, therefore, pipes and joints are supposed to descend into the ground while the high grease pressure remains unattenuated. The high grease pressure sets up a circumferential stress of 40 kg/mm.sup.2 or above in the meshed box. The circumferential stress increases as the liquid passing through the pipes exerts pressure from inside. Consequently, the box, if made of weak materials, may break in such sulphide-polluted environment. While, in addition, the box 1 expands, the pin 3 contracts as shown in FIG. 1. Then, the metal-to-metal contact seal 5 fails to maintain large enough sealing pressure, which often causes leakage from the joint. With conventional joints, joint stress is sometimes lowered by selecting an appropriate amount and method of grease application. But such control is too intricate and unreliable to be applied on the field.
The aforementioned problem exerts particularly profound influence on tubing (23/8 inches to 7 inches in diameter) and casing (51/2 inches to 11 inches in diameter) of the type required to have high sour-corrosion resistance and sealing ability.